1. Field of the Invention
This invention relates generally to pistons for internal combustion engines, and more particularly to diesel pistons having internal oil cooling features.
2. Related Art
Hollow piston constructions for diesel engines are known for providing enhanced cooling capabilities, which in turn often yield improvements in exhaust emissions and extended service life. In these applications, the normal engine lubricating oil is used to help cool (convectively) the hot head, or specifically the under-crown region, as well as the outer ring belt region of the piston. In some hollow piston configurations, a single outer cooling gallery near the ring belt region may be used, or a central oil gallery under the crown region, or two galleries paired (dual galleries) in various combinations of open and closed geometries. Dual gallery pistons typically have an annular, radially outer cooling gallery and an open central cooling gallery formed between upper and lower crown portions. The outer and central galleries can either be isolated from one another or arranged in relatively open fluid communication with one another via multiple oil passages extending through intervening ribs. In addition, it is known to provide pin lubrication passages extending from one or both of the galleries to a wrist pin. The lubrication passages can, for example, extend into a wrist pin bore of a pin boss and/or between laterally spaced pin bosses. The outer gallery, whether formed as a single or dual gallery construction, is particularly suited for cooling a ring belt region of the piston, while the central gallery, if present, is particularly suited for cooling a central crown region formed in part by a combustion bowl wall or dome, which is directly exposed to hot combustion gasses.
The combustion dome and underlying central crown region (i.e., under-crown) are exposed to extreme heat in use. Without proper management of heat in this under-crown region, several problems can result. For example, it is possible that carbon build-up on the under-crown will form over time. This carbon build up will further reduce the heat transfer from the combustion bowl leading to higher temperatures on that region. This carbon build up can eventually flake off. Loose carbon flakes can be caught between moving components and cause scratches. Another problem associated with excessive heat build-up in the under-crown region relates to exhaust emissions. If combustion temperatures are not tightly controlled in diesel engines, the combustion process can not be optimally regulated for efficiency and emissions concerns. And further, if the piston temperatures are allowed to rise too high, the lubricating oil can become over-heated and begin to chemically break down prematurely, thus reducing its service life.
Over the years, engine designers have sought to provide sufficient oil flow in the central crown region while at the same time avoiding deterioration of the oil due to over-heating to avoid the aforementioned problems. If an insufficient supply of oil is directed to the under-crown region, or if the oil is allowed to remain in the region for too long, the oil over-heats and its cooling and lubrication functions are diminished. As such, an ample flow of cooling oil must be provided in order to properly regulate the temperature of the under-crown region.
There is therefore a need in the art for improved temperature management strategies in piston design, and in particular for the design of diesel pistons, that optimally cools the under-crown region with lubricating oil during use.